A liquid material (such as a dispersion containing a fluororesin, a dispersion containing a fluororubber, a solution containing a fluorosurfactant or a waste liquid discharged in a production step) to be treated for production of a fluorinated compound (such as a fluororesin, a fluororubber or a fluorosurfactant), contains a slight amount of fluoride ions.
In an equipment or an apparatus for treating a liquid material containing fluoride ions, a metal is corroded by the fluoride ions, at a portion to be in contact with either one or both of the liquid material containing the fluoride ions and a condensate of gas vaporized from the liquid material. Further, a liquid material to be treated for producing some types of fluorinated compounds may be treated under acidic conditions with hydrochloric acid. Therefore, in a case where a liquid material containing fluoride ions is treated particularly under acidic conditions with hydrochloric acid, it is necessary to carry out lining (such as glass lining or resin lining) to the surface of a metal at a portion to be in contact with the liquid material or the condensate, or to employ carbon fiber reinforced plastics (hereinafter, referred to as “CFRP”) instead of a metal.
In the case of the resin lining, if the liquid material containing fluoride ions further contains an organic solvent, the organic solvent may penetrate between resins or between a resin and a metal, whereby peeling (hereinafter, referred to as “blister”) may occur to the resin. Particularly in e.g. a polymerization tank or a heat exchanger, if a heat medium having a large temperature difference with the liquid material containing fluoride ions is made to flow into a jacket side of the polymerization tank or a shell side of the heat exchanger, blistering tends to occur. Therefore, in a case where a liquid material containing an organic solvent is treated in an equipment or an apparatus lined with the resin, operation conditions are significantly restricted.
In the case of employing CFRP, an equipment or an apparatus employing the same may increase in cost. Further, depending upon the type of the liquid containing fluoride ions, phenol derived from an epoxy resin used for CFRP may be eluted into the liquid material. If a fluorinated compound contained in the liquid material and phenol eluted into the liquid material have a small difference in boiling point, it is difficult to separate the phenol by distillation.
The glass lining, which is corroded by fluoride ions, is not suitable for an equipment or an apparatus for treating a liquid material containing fluoride ions. Nevertheless, the resin lining and CFRP have the above-mentioned problems, and therefore there is no way but to employ the glass lining even when a liquid material containing fluoride ions is used.
As a method for suppressing corrosion of glass lining, the following methods are well known.
(1) A method of lowering a temperature of a liquid material containing fluoride ions and a temperature of an equipment or an apparatus, as far as possible.
(2) A method of preventing inclusion of moisture into a liquid material containing fluoride ions.
(3) A method of suppressing a concentration of fluoride ions in a liquid material, to be 2 mass ppm or less.
(4) A method of controlling a pH of a liquid material containing fluoride ions, to be within a range of more than 4 and less than 7.
(5) A method of reducing a solid content in a liquid material so as to avoid handling of slurry fluid.
However, operation conditions are still significantly restricted in the case of any of the methods (1) to (5).
As other methods for suppressing corrosion of glass lining, the following methods may be considered.
(6) A method of adding an alkaline substance (such as CaCl2 or NaOH) to a liquid material containing fluoride ions so as to precipitate the fluoride ions as a salt (such as CaF2 or NaF), separating the salt from the liquid material, and then treating the liquid material in an equipment or an apparatus.
However, according to the above method (6), an aimed fluorinated compound contained in the liquid material may not only decrease in yield, but also be hardly recovered, if the aimed fluorinated compound contained in the liquid material is reacted with an alkaline substance or if the liquid material is treated under acidic conditions.
As other methods for suppressing corrosion of glass lining, the following method may be proposed.
(7) A method of controlling the upper limit of a fluoride ion concentration, in accordance with a pH of the liquid material containing fluoride ion and a temperature of the liquid material or a heating medium (Patent Document 1).
However, operation conditions are significantly restricted in the case of the method (7). In particular, if a pH is 1 or less and a temperature is 20° C., this method cannot be applied unless the fluoride ion concentration is 7 mass ppm or less. Further, if a pH is 3 and a temperature is 20° C., this method cannot be applied unless the fluoride ion concentration is less than 290 mass ppm. Further, this method fails to define conditions of a fluoride ion concentration in a case where a pH is less than 1.